内径 and 外径: inner and outer diameters
Distinguish the inner opening from the outer boundary of a tube. Values in millimeters still describe different diameters when labeled 内径 and 外径.
内径 measures the opening
内径, read ないけい, means inner diameter. 技師は管の内径を専用の器具で測った。 says that the technician measured the tube’s inner diameter with a specialized instrument.
For a hollow circular tube, the inner diameter is the diameter of the inner circle. It spans the opening between its inner boundaries, rather than the tube’s entire outer width.
外径 measures the outer boundary
外径, read がいけい, means outer diameter. 管の外径を測って合う部品を選んだ。 says that someone measured the tube’s outer diameter and chose a part that fit.
A tube with an inner diameter of 8 millimeters and an outer diameter of 10 millimeters has two measurements of different boundaries. Sharing a unit does not make those measurements interchangeable.
Which diameter matches the drawing?
部品の内径が設計図の数値と一致している。 says that the part’s inner diameter matches the value in the design drawing. The item being checked is the inner diameter.
Matching the inner diameter does not establish that the outer diameter has also been checked. Keep the label 内径 or 外径 with each value when comparing parts.
If a tube has an inner diameter of 8 millimeters, can you say its outer diameter is also 8 millimeters?
Show the answer and why
No. The inner diameter measures the opening. The outer diameter is a separate measurement labeled 外径.